1. Field of the Invention
This invention relates to automatic focusing adjustment devices and, more particularly, to automatic focusing adjustment devices using the so-called difference active type range finder in which light is projected onto an object to be photographed and its reflection is passed to the area-divided photosensitive element.
2. Description of the Prior Art
There have been many previous proposals for automatically adjusting the position of the photographic lens or image pickup device in accordance with the object distance measured by the range finder so that an image of an object to be photographed is brought into sharp focus.
Of these, the typical one is that light of particular pattern is projected onto the object, and the reflected light from the object is detected by two image receiving areas of a sensor, wherein the object distance is determined based on the difference between the outputs of the two areas, or the so-called difference active type range finder is employed. Although the accuracy of focusing adjustment is relatively high even though the structure is simple, this type of range finder has a problem in that the reliability is considerably lowered for objects of differing reflectivities from part to part, or of differing contrast.
This problem will next be analyzed by reference to the drawings. FIGS. 1 and 2 illustrate a camera equipped with the prior known automatic focusing adjustment device employing the difference active type range finder. Near infrared light from a semi-conductor laser or like light emitting element 4 is projected by an aspherical molded lens 3 onto an object 1 where the projected light takes a spot-like form. This light spot is reflected from the object 1 to an aspherical molded collection lens 5 by which an image of the spot is formed on a sensor 6, for example, SPC, having two image receiving areas 6a and 6b.
In the in-focus condition as shown in FIG. 1, the center of the round spot image on the sensor 6 takes its place at the boundary line between the two areas 6a and 6b. Letting A denote the output of the first area 6a and B the output of the second area 6b, we have A-B=0.
Upon detection of when the outputs A and B coincide with each other (in actual practice their difference falls below a certain level), an image of the object 1 on the film plane, image pickup tube, or device is determined to be sharply focused.
As the object 1 moves toward the camera, the spot image on the sensor 6 shifts toward the area 6b as shown in FIG. 2. After the outputs A and B have been amplified and integrated by a signal processing circuit 7, when the former is subtracted by the latter, the difference between the outputs A and B is found negative, indicating a far-focus condition. A microcomputer 8 controls the operation of an electric motor 9 in accordance with the magnitude and sign of the output of the signal processing circuit 7 so that a focusing component 2 of the photographic lens along with the light emitting element 4 and sensor 6 moves forward or in a direction indicated by the arrow. Then, when A-B=0 is reached, the motor 9 stops. Thus, the in-focus condition is established again with respect to the new object distance. For note, in actual practice, by considering the presence of noise, there is provided a blind zone K permitting .vertline.A-B.vertline.&lt;K to be taken as being in-focus.
FIG. 3 illustrates another example of the prior known difference active type range finder employed in a different type camera from that of the camera of FIGS. 1 and 2. While the optical system of the range finder of FIGS. 1 and 2 is entirely independent of the photographic optical system, the light projection optical system of FIG. 3 is constructed with common parts of the photographic optical system. this provides the advantage that the camera of FIG. 3 is generally superior in compactness to that of FIGS. 1 and 2.
The light source 4 is positioned at an optically equivalent point to the axial point on the focal plane of the photographic lens system. Therefore, a sharp image of the light source 4 is formed in the object space at a distance to which the focusing member 2 is focused. Rays of light from the light source 4 are collimated by a lens 3 and then reflected by a dichroic mirror 13 which reflects only a nonharmonic region of wavelengths to the ambient light. A light beam from the mirror 13 has its axis in coincidence with the optical axis and passes through a zoom section 12 and the focusing component 2, being projected into the object space. While, in the example of FIGS. 1 and 2, the light source 4 (or the projection lens 3 as the case may be) is made movable along with the focusing lens 2, this is not necessary in the example of FIG. 3. For note, R0 in FIG. 3 denotes the distance in the base line of the trigonometrical survey.
FIG. 4 illustrates still another example of arrangement of the difference active type range finder relative to the photographic lens of the camera. In this case, the light source 4 and sensor 6 are positioned at respective optically equivalent points to the axial point on the focal plane of the photographic lens 2.
All the above-described kinds of range finders of the difference active type have a common problem that the distance measurement operates with decreasing reliabilities as the range of reflectivities of the object increases. Taking the example of FIGS. 1 and 2 as representative, this problem is next explained in greater detail.
The range finders of the difference active type have generally so far been designed on the major premise that the geometrical center of the light spot image on the sensor coincides with the center of power.
It is, however, obvious that the sensor 6 produces different outputs for objects of different distribution of reflectivity, despite the fact that these objects lie at an equal distance to one another. If that area of the object which is illuminated with the projected light is uniform in reflectivity, therefore, no particular problem arises. But, if the distribution of reflectivity over that area is not uniform, the aforesaid rule of design can no longer stand. In this connection, discussion will next be made by reference to FIGS. 5 and 6.
In FIG. 5, an object 10 has its upper half surface of high reflectivity and its lower half surface of low reflectivity, and the photographic lens is assumed to be initially just focused on that object. When the automatic focusing adjustment device is then rendered operative, however, an out-of-focus signal is produced, because A-B&lt;0. As a result, the motor 9 is energized to move the focusing lens component 2, light source 4 and sensor 6 in a direction indicated by the arrow. In more detail, the reflected light from the high reflective upper half of the illuminated area on the object 10 falls in the second domain 6b, and that from the low reflective lower half in the first domain 6a. Although the geometrical center of the spot image on the sensor 6 takes its place at the boundary line between the domains 6a and 6b, their outputs differ from each other as B&gt;A, causing the range finder to give a signal representing a far-focus condition to the automatic focusing adjustment device.
FIG. 6 illustrates an automatically adjusted focusing position where A-B=0 in contradiction to the fact that the geometrical center of the area of the spot image on the sensor 6 lies in the domain 6a. Thus, the apparent object distance y2 is shorter than the true one y1.
The use of such a conventional range finder of the difference active type in the automatic focusing adjustment device led to a disadvantage of lowering the reliability of focusing adjustment for so-called contrast pattern objects. This disadvantage can be reduced to some extent by decreasing the size of area of the light spot, but cannot be brought to naught, since there is a fundametal background. Also, if the size of the area of the light spot is unduly largely descreased, an alternative problem arises that when actually built in the camera, the distance adjusting ring is caused to excurse very frequently, although the precision accuracy of distance measurement is improved.
Another method of reducing the aforesaid disadvantage is to increase the base line length of the trigonometrical survey, or the distance between the light source and the sensor. Since the light source and the sensor are arranged so as to increase the base line length until that disadvantage becomes negligible, as the manageability and the flexibility of design must be largely sacrificed, this method has little value in actual practice.
How much discrepancy is produced between the actual and measured object distances for the contrast pattern object is next considered, for it constitutes the principle of the present invention.
In FIG. 7(b), an infrared spot image 101 on the sensor is assumed to have a radius of unity. A hatched or left hand half and slightly more area of the spot image 101 comes from that portion of the object which has a unit of reflectivity for the infrared light, while a right hand or white area of the circle 101 represents a larger value, k, of reflectivity than unity (1&lt;k). (The term "value of reflectivity" herein used means that this value is measured in respect to the wavelength of the projected light). The abscissa is in the distance of a line, l, at which the intensity of light changes from the center 0 of the circle 101 as an original point. G denotes the center of light intensity. The integrated light intensity over that area of the circle 101 which lies on the left hand side of a vertical line passing the point G is equal to that over the other area on the right hand side. The distance from the center 0 of the circle 101 to the center G of light intensity varies as a function of deviation of the line, l, from the geometrical center 0. In the case of k=8, this function is depicted in FIG. 7(a). This curve tells that when the position of the brightness boundary line l comes to 0.6, the distance E takes a maximum value of about 0.7. As a matter of course, for l=1, or l=-1, that area of the object which is illuminated with the projected light is uniform in reflectivity, and, therefore, E=0. Accordingly, the contrast dependent defocused quantity can be evaluated in terms of E.
Here, let us relate various points in the curve of FIG. 7(a) to the real situations. In FIG. 7(c), a circular infrared spot image 101 is obtained from an object of uniform high reflectivity. Because the intensity of light is uniformly distributed over the entire area of the circle 101, the center of the circle 101, or the geometric center 0 of the spot image coincides with the center of light intensity G1, or the distance between the geometrical center 0 and the center G1 of light intensity takes a value E1 of zero.
In FIG. 7(d), as the boundary line of reflectivity enters the spot of illumination, for the line l is positioned at -0.5 with the hatched area of low light intensity and the remaining or white area of high intensity, the center of light intensity G2 splits from the geometrical center 0 to the right by a distance E2.
As the line l further invades to l=0 and l=+0.5 shown in FIGS. 7(e) and 7(f) respectively, the center of light intensity G is shifted progressively farther from the geometrical center 0, taking values G3 and G4 respectively with the distance E at E3 and E4.
Then when all the area of the circle 101 is uniformly low in light intensity, as shown in FIG. 7(g), the reflection signal strength center G5 shifts backward to coincide with the geometrical center 0 with the distance E becoming zero again (or E5=0). By plotting the values of distance E1 to E5 in the ordinate with respect to the values of l, and connecting the points successively, the curve of FIG. 7(a) is obtained.
How the contrast dependent defocused quantity E affects the output of the sensor when the number of image receiving areas is two is next discussed by reference to FIGS. 8(a) and 8(b).
When an infrared light spot image which is first assumed to be of uniform intensity sweeps the domains A and B of the sensor past positions (a), (b) and (c) successively, (VA-VB)/(VA+VB) where VA and VB are the outputs of the domains A and B of the sensor respectively varies as shown by solid lines in FIG. 8(a). When the geometrical center 0 of the spot image 101 comes across the boundary line between the domains A and B of the sensor, (VA-VA)/(VA+VB) (hereinafter referred to as AF signal) becomes zero. Therefore, no focusing misadjustment by contrast is formed.
For another spot image of contrast pattern with l.apprxeq.0.6 and k=8, as shown in FIG. 8(b), variation of the value of the AF signal with variation of the position of the spot image past (a).fwdarw.(b).fwdarw.(c) is shown by a dashed curve in FIG. 8(a). This curve crosses the X-axis at a distance .DELTA.x from the original point, the cross point corresponding to the position (b) of the spot image in FIG. 8(b). It is to be noted here that the signal strength center G lies just on the boundary line between the domains A and B of the sensor. This means that E of FIG. 7(a) corresponds to .DELTA.x of FIG. 8(a). That is, the AF signal becomes zero when the spot image is deviated by E. The defocused amount, .DELTA.b, on the focal plane may be expressed by .DELTA.b=.DELTA.x.multidot.f.sup.2 /(L.multidot.fs) where f is the focal length of the photographic lens, L is the base line length of trigonometrical survey and fs is the focal length of the collection lens. The range of focusing misadjustment is proportional to .DELTA.x, and increases with increase in the contrast rate k, as the maximum value of FIG. 7(a) moves to the right upward.